Gum trees are causing headaches for mobile phone providers in rural areas because they interfere with the signal, a telecommunications company announced this week.

Country Wide, Telstra's rural arm which recently celebrated its second year, has found that groves of trees break mobile telephone signals.

And because gum trees are the most conspicuous trees in the country, they are being blamed.

"It has been a known fact for a long time that trees interfere with reception either by absorption or reflection," said Mr Roger Bamber, an engineer and Country Wide's NSW managing director. "It has been a problem since the 1950's and 1960's."

"I think the fact is we have a lot of eucalypts in Australia. Most of the corridors on rural roads are eucalypts," he said. "We think it is the size, shape and moisture content of the eucalypt leaves that absorbs or reflects the signal."

"Trees of any sort have been known to block radio signals - they both absorb the signal and scatter it in many directions."

Dr Young's team have conducted measurements in rural areas and have recorded lower signals when they passed trees. "I don't think it's peculiar to gum trees; there is no evidence that gum trees are worse than introduced trees," he said.

It does appear, however, that moisture is the culprit. A dead tree absorbs fewer signals than a live tree without leaves, which absorbs less than a tree with leaves. Live trees with leaves that have rain on them absorb the most signals.

This is a problem that particularly affects rural customers. "In capital cities trees are not the major problem," said Dr Young. "Signals are more likely to be affected by interference from local traffic."

The problem came to Telstra's notice when people boating on the Murray River in southern NSW complained that they were losing the signal. The region is flat and well serviced by mobile phone towers. There should not be any problems for mobile phones. The area is, however, surrounded by dense stands of river red gums.

Trees cause more of a problem for higher frequency signals, such as radar and LMDS, than for mobile phone signals which transmit at less than 900Mhz. The issue for mobile phones is the number of trees that are between the signal point and the handset. "At around one to two GHz the effect of trees is relatively small," said Dr Young. "But once you get enough of them they can certainly block the signal."

Mr Bamber encourages people to use the external antenna in their car kits to get higher gain if they are going to be driving in rural areas.

Telstra anticipates this problem will also affect the satellite telephone network, being introduced soon. "Satellites need a clear line of sight, and eucalypts can affect that," said Mr Bamber.

All radio services at higher frequencies use satellite communication. "They need to see a lot of the sky to work because we are using a low earth-orbiting satellite," said Mr Bamber. "So if you are between two buildings, in a very dense grove of trees or in a deep ravine, you might not have continuous sighting of the satellite."

Because satellites are low powered they emit only what you need plus a little bit more, said Dr Young. The satellite signal comes in from a high angle so trees are only really a problem when the signal is low in magnitude.